| In the past few decades,Li-ion batteries have rapidly developed and have been widely used in small portable electronic devices.To meet the needs of electric vehicles and large-scale smart grids for Li-ion batteries with high energy density and high cycle stability,on the one hand,it is necessary to synthesize electrode materials with excellent performances,on the other hand,it is necessary to understand the charging and discharging mechanism of batteries and explore effective ways to improve the electrochemical activity and structural stability of electrode materials.Layered Li-rich Li2 Mn O3,cation-disordered Li-rich oxide,and spinel-like Li Ni0.5Mn1.5O4 have attracted much attention due to their high-energy density,abundant reserves,non-toxicity,and simple preparation process and have become a very promising electrode material in the future.However,in practical applications,the above materials all have certain limitations.First-principles calculations have become a useful tool for designing new electrode materials,revealing the origin of the electrochemical activity of materials,and screening appropriate dopants to improve the performance of electrode materials.Our work takes layered Li-rich Li2 Mn O3,cation-disordered Li-rich oxide and spinel-like Li Ni0.5Mn1.5O4 as the research objects,basing on first-principles calculations,explore their electrochemical mechanism,improve their structural stability and electrochemical activity by optimizing dopants,establish the relationship between structures and electrochemical properties,and provide a theoretical reference for the experimental research and development of high-performance Li-ion battery cathode materials.The main research contents and conclusions of this paper are as follows:1.Theoretical calculations explored the relationship between oxygen vacancies and the electrochemical activity of layered Li-rich Li2 Mn O3 in different delithiation states,and found an effective doping way to improve the structural stability of Li2 Mn O3.Li2 Mn O3 will inevitably introduce oxygen vacancies in the process of crystal structure synthesis,however,the contribution of oxygen vacancies to the structural stability of Li2 Mn O3 during the charging and discharging process still needs to be systematically explored.The formation sites of oxygen vacancies in the layered Li-rich Li2 Mn O3were determined by calculating the defect formation energy.Comparing the energy barriers of Mn ion migration from the initial site to the Li layer for intrinsic Li2 Mn O3and the Li2 Mn O3 with oxygen vacancy under different delithiation states,it is found that the influence of the introduction of oxygen vacancy(VO)on the phase transition of Li2 Mn O3 is closely related to the delithiation amount: at light delithiation(Li1.5Mn O3),VO inhibits the phase transition of the structure.While under moderate and heavy delithiation(Li Mn O3 and Li0.5Mn O3),VO promotes the phase transition of the structure.The former is mainly due to the reduction effect of the introduction of VO on Mn,and the latter is mainly due to the change of the coordination environment around Mn induced by the introduction of oxygen vacancies.In order to further improve the structural stability of Li2 Mn O3,we studied the doping of a series of transition metal elements and found that the substitutional doping of Mn by V can effectively inhibit the structural phase transition of Li2 Mn O3 during the delithiation process.2.The ground state structures,transition metal(TM)valence states and Li ion migration energy barriers of Li2Mn0.5Ti0.5O3 with different fluoride doping contents were studied,and the relationships between fluoride doping content and electrochemical activity was theoretically revealed.It has been confirmed that fluorine doping improves the cycle stability of cation-disordered Li-rich oxides.However,the electrochemical performance of materials varies significantly with different fluorine doping contents.Theoretical calculations found that for Li2Mn0.5Ti0.5O3-x Fx,F atoms are more likely to dope into the Li-rich region and preferentially coordinate with Mn.When the F doping content x ? 0.5(light doping content),the coordination of Mn atoms changes from undoped Mn O6 to Mn O5 F,and at the same time,the Mn coordinated with F atoms are reduced from the original +4valence to +3 valence,the distance between Mn and Li in the center of the tetrahedron is almost unchanged,and the bonding angle between Mn and the nearest neighbor anion decreases with the increase of F doping content,which leads to the reduction of Li ion migration energy barrier.When the F doping content 0.5 < x ? 1(heavy doping content),the newly doped F atoms no longer coordinate with Mn,but with Ti,the electrons provided by the fluorine atoms will still transferred to the Mn atoms through the Ti atoms,thereby the +3 Mn ions are further reduced to +2 valence.Under this F doping content(heavy fluorine doping),the distance between Mn and Li in the center of the tetrahedron and the bonding angle between Mn and the nearest neighbor anion significantly increase with the increase of F doping content,which becomes the main reason for the increase of the Li ion migration energy barrier.During the whole process of F doping(0 < x < 1),Mn always act as the electrochemical active center during charge and discharge,increasing the theoretical capacity provided by Mn,inhibiting the dimerization of oxygen to a certain extent and delaying the release of oxygen,thus inhibits the degradation of the structure during the delithiation process.In addition,through a seriers of structural comparisons and analysis,we found that the valence state of TM in the Li ion transfer channel,the distance between TM and the Li in the center of the tetrahedron,and the arrangement of anions are the major factor which affecting the Li ion transfer in the cation-disordered Li-rich oxide.3.The physical mechanism of Cr doping induced the phase transition of Li Ni0.5Mn1.5O4 from ordered phase to disordered phase and the origin of doping improving the transport property of Li ions were theoretically explored.The structural stability of spinel-like Li Ni0.5Mn1.5O4 can be improved by doping the main group or transition metal elements.Among them,Cr doping especially improves the structural stability of Li Ni0.5Mn1.5O4.Experiments show that Li Ni0.5Mn1.5O4 is an ordered phase after annealing,wheaers after Cr doping,the Li Ni0.5Mn1.5O4 will undergo a phase transition from ordered phase to disordered phase.This process not only increases the structural stability of Li Ni0.5Mn1.5O4,but also improves the transport performance of Li ions.In this paper,we can determine that Cr prefer to substitute Ni in Li Ni0.5Mn1.5O4 rather than Mn by calculating defect formation energy,the disordered phase Li Ni0.5Mn1.5O4 with Cr doping has lower energy than the ordered phase Li Ni0.5Mn1.5O4 with Cr doping,and the energy is reversed comparing with the intrinsic ordered and disordered Li Ni0.5Mn1.5O4,which is consistent with the experimental observation of the Cr doping-induced phase transition from ordered phase to disordered phase.Cr doping induces the reduction of Mn ions in the Li Ni0.5Mn1.5O4.The reduced Mn ions not only reduce the diffusion barrier of Li ions inside the Li Ni0.5Mn1.5O4,facilitating the migration of Li ions in the structural framework,but also reduce the formation energy of Li ion vacancies,facilitating the extraction of Li ions from the structural framework.In addition,Cr doping increases the structural stability of Li Ni0.5Mn1.5O4 during the delithiation process,which is beneficial to reduce the strain of the structure during charging and discharging. |
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